An exhaust gas discharged by an internal combustion engine of a vehicle or the like contains hazardous gases such as carbon monoxide (CO), nitric oxide (NOx), and unburned carbon hydride (HC). In general, a palladium catalyst is used as a catalyst for exhaust gas purification (a so-called ternary catalyst) in order to degrade such hazardous gases, because palladium is less expensive than rhodium. However, in terms of heat tolerance, NOx purification performance, etc., palladium catalysts are inferior to highly active rhodium catalysts, which has been problematic. Moreover, since palladium is a noble metal, there is a need to improve activity of palladium catalysts in order to reduce the consumption of palladium itself.
In addition, ternary catalysts for diesel or lean-burn engines have been conventionally developed. However, control of gasoline vehicles is adjusted within a range of stoichiometric atmospheres. Meanwhile, it is predicted that the use of fuel cut-off (F/C) means for gasoline vehicles will increase in the future for the improvement of fuel efficiency. In this regard, performance improvement in a lean atmosphere would become more important. Therefore, there is a demand for ternary catalysts capable of desired activity in environments with different air-fuel ratios (A/F), such as a lean atmosphere, a stoichiometric atmosphere, or a rich atmosphere.
Patent Document 1 discloses a method for supporting a polymer chelated metal catalyst, comprising a step of supporting a composite metal colloid on a carrier, the composite metal colloid being obtained by forming chelate bonds between at least two different catalyst metals using a chelator. Patent Document 1 also discloses that a specific chelator is used for improving the dispersibility of barium because sufficient NOx storage capacity cannot be obtained when barium is supported on a catalyst comprising a polyvinylpyrrolidone platinum/rhodium composite colloid.
Patent Document 2 discloses a catalyst for exhaust gas purification, which is formed with a noble metal and a base material supporting the noble metal. In addition, Patent Document 3 discloses a catalyst for exhaust gas purification, which comprises a porous carrier that supports palladium and iridium, palladium and iridium being formed into an alloy on the porous carrier.
Meanwhile, neither Patent Document 2 nor 3 discloses that catalyst metal particles can be obtained by treating a colloidal solution containing two different noble metals such that the noble metals are reduced at the same time.
Conventional catalysts for exhaust gas removal are specialized for the reduction of NOx and the improvement of quality of water vapor of HC. This means that sufficient purification performance for all of CO, NOx, and HC cannot be expected from the conventional catalysts.
Accordingly, it has been required to develop a palladium catalyst having further improved activity that can be used within a wide range of atmospheres.